Igneous Rocks + Volcanoes (Week 3)

Rock

Solid aggregate of minerals and sometimes mineral-like substances (organic matter and glass)

Classes of rocks

• Igneous

• Sedimentary

• Metamorphic

Importance of Rocks

• Contains minerals

• Serve as aquifers (storage systems for groundwater) 

• Contain almost all the world’s petroleum (for fuel and manufacturing)

• Contain world’s mineral resources

• Record Earth’s history 

The rock cycle 

Depicts dynamic changes that take place on and within Earth to transform classes of rocks

Igneous → Sedimentary

Erosion + Deposition 

Sedimentary → Igneous

Melting + Crystallization

Igneous → Metamorphic 

Heating + Pressing 

Metamorphic → Igneous

Melting + Crystallization

Metamorphic → Sedimentary

Erosion + Deposition

Sedimentary → Metamorphic 

Heating + Pressing 

Characteristics of Rocks 

• Composition (mineral/fossil/organic content) 

• Texture (arrangement of crystals or particles) 

• Additional geological structures within the rock

Function of characteristics of rocks

To indicate and reconstruct the history of the rock

Igneous

“Born of fire” - begins as magma and rises toward Earth’s surface in magma chambers

Formation of Extrusive Igneous Rocks 

Magma rises and cools QUICKLY on or near the Earth’s surface via volcanoes 

Formation of Intrusive/Plutonic Igneous rocks

Magma trapped below earth cools SLOWLY below the surface

How Igneous rocks are classified

Texture (which records cooling history)

Mineral composition (indicates how magma was formed)

Igneous texture

Indicates cooling history

• Slow cooling below Earth’s surface creates large crystals

• Rapid cooling at/above Earth’s surface creates tiny crystals 

• Quenched (essentially immediate) cooling at Earth’s surface creates no crystals

Igneous mineral composition 

Provides information on how magma was formed 

• Magma formed and cooled at high temperatures are rich in FERROMEGNESIAN SILICATE minerals (are dark)

• Magma formed and cooled at low temperatures have less ferromagnesian silicate minerals (are light) 

Phaneritic Texture

Mineral crystals visible to unaided eye, cooling was slow and at great depth (allowing crystal formation)

Aphanitic Texture

Mineral crystals too small to be visible by unaided eye, cooling was fast at or near the surface (crystals had little time to grow in lava)

Glassy Texture

Result of quenching (immediate cooling) at surface, no time for crystals to grow

Igneous Texture and Cooling History - Extrusive

Extrusive igneous rocks crystallize rapidly to produce a APHANITIC/GLASSY texture

Igneous Texture and Cooling History - Intrusive

Intrusive igneous rocks crystallize slowly at depth to produce PHANERITIC texture

Principle composition of most igneous rocks

Silicate minerals - ferromagnesian and non-ferromagnesian

Ferromagnesian silicate-rich igneous rocks

Tend to be darker in colour

Non-ferromagnesian silicate igneous rocks

Tend to be lighter in colour 

Common Ferromagnesian Silicate Minerals of Igneous Rocks 

• Olivine 

• Pyroxene 

• Amphibole 

• Biotite mica 

Common Non-ferromagnesian Silicate Minerals of Igneous Rocks

• Feldspars, which have silica framework and light cations like K, Na, Ca (Plagioclase and Orthoclase) 

• Quartz (very common, consists of only silica)

note Ca rich plagioclase is dark coloured 

Bowen’s Reaction series: Ferromagnesian silicate minerals

Discontinuous sequence

Olivine → Pyroxene → Amphibole → Biotite Mica

Bowen’s Rection Series: Non-Ferromagnesian silicate minerals

Continuous sequence

Ca-rich feldspar → Na-rich feldspar → K-rich feldspar → Muscovite Mica → Quartz

Colours in Bowen’s Reaction Series

Minerals cooling at higher temperatures: Darker

Minerals cooling at higher temperatures: Lighter 

Igneous Compositions

Described the composition of igneous rocks to approximate proportions of light-dark materials 

• Felsic

• Intermediate

• Mafic

• Ultramafic

Felsic (granite)

Predominantly light-coloured minerals 

• Feldspar and quartz

Intermediate (diorite)

Equal light and dark minerals 

Mafic (gabbro)

Predominantly dark coloured minerals 

• Containing Mg and Fe

Ultramafic (olivine)

Predominantly dark-ish coloured minerals 

Naming Igneous Rocks

Composition + Texture

Felsic + Phaneritic

Granite

Felsic + Aphanitic

Rhyolite 

Intermediate + Phaneritic

Diorite

Intermediate + Aphanitic

Andesite

Mafic + Phaneritic

Gabbro

Mafic + Aphanitic 

Basalt 

Problem with melting rock

Temperature increases at depth (atoms apart), pressure increases at depth (atoms together)

Plate tectonics

Consists of Earth’s lithosphere broken up and floating on the asthenosphere, concept is them moving away, toward, or alongside one another

Magma formation: Decompression melting 

• Generated where plates are diverging 

• Produced by melting of low and high temperature minerals (able to melt because near the surface, so pressure is low)

Magma Composition: Decompression melting

Contains silica, and a large amount of Fe and Mg 

Rocks formed: Decompression melting

Mafic rocks

• Intrusive: gabbro

• Extrusive: basalt

Magma Formation: Hydration melting

• Generated by converging plates

• Formed at lower temperatures than diverging plates, water released from descending plate turns to steam and lowers melting point of mantle rock

• Low temperature minerals are melted 

Magma Composition: Hydration melting

• Rich in silica, can be further enriched in silica by silica-rich rocks in the crust 

Rocks formed: Hydration melting

Intermediate to felsic rocks: 

• Intrusive: diorite or granite 

• Extrusive: andesite or rhyolite 

Intrusive igneous bodies 

• Features formed by the ascent of magma (through a stack of sedimentary strata) 

• Magma cools and crystallizes into solid igneous rock 

• All rocks have phaneritic texture

How magma can ascend through strata

• Rise through conduit of volcano

• Intrude between strata (run parallel)

• Intrude into subvertical fractures (cut across strata)

Laccolith 

Domed, blister-like body 

Volcanic neck/plug

A “plug” of igneous rock

Dike

Subvertical sheet-like body that cuts through strata

Sill

Sheet-like body parallel to strata

Products of volcanic eruptions

1. Lava

2. Pyroclasts

3. Volatiles

Pyroclastic material

Airborne blobs of lava and pulverized bits of rock

• blobs of lava land as volcanic “bombs”

Volatiles 

Gases that have escaped from magma during eruption 

Lava

Liquid molten rock

Factors affecting eruption style

• Composition of magma

• Temperature of magma

• Dissolved gases in magma

Viscosity

Controlled by factors affecting eruption style AND the nature of an eruption 

Convergent boundaries

• Magma produced by hydration melting

• Melted rock is enriched in silica

• Intermediate to felsic composition

Divergent boundaries

• Magma produced by decompression melting

• Melted rock is silica-poor

• Mafic composition

Intraplate hotspots

• Independent of plate boundaries

• Discrete heat plumes that locally melt high-temperature asthenosphere rock

• Magma has mafic composition

  • Continental areas have magma contaminated by molten continental crust, leading to a more felsic composition

Main texture of lava rock

Aphanitic (due to rapid cooling)

Most important volcanic rocks

• Basalt (mafic)

• Andesite (intermediate)

• Rhyolite (felsic)

Formation of pyroclastic materials 

• Blockage of volcanic vent cooled by lava: lava dome produced 

• Lava dome explodes due to release in pressure

• Pulverized materials release (drastically ranging in size)

Special textures of volcanic igneous rocks

• Porphyritic

• Glassy

• Vesicular

• Pyroclastic

Porphyritic Texture

• Aphanitic component with phenocrysts (large crystals)

• Has a two stage cooling history

  • Magma cooled slowly at depth (allowing larger crystals to form)

  • Magma rose quickly to the surface (remaining liquid cooled quickly to produce tiny crystals in fine-grained “ground mass”

Glassy Texture 

• Surface of lava that is quenched (cooled very quickly), ions could not produce crystals 

• High silica content also slows down movement of ions

• Ions remain disordered and produce obsidian 

Vesicular texture 

• Presence of voids preserved in volcanic rock

• Laval becomes frothy and is full of holes in extreme cases (so light it floats) 

  • Has 2 types

Scoria

Mafic + vesicular

Pumice

Felsic + vesicular 

Pyroclastic flow

• Common in explosive eruptions

• Cloud of hot pyroclastic material (mostly ash) and gas forced to flow downhill 

Nuee Ardente

• Growing cloud

• Pyroclastic flow so hot that it glows

Pyroclastic texture

• When material settles from pyroclastic flow

• Angular, glassy fragments are welded together

• Tuff is produced

Volatile content 

• related to explosive tendency 

• Higher content increases tendency for magma to be explosive (producing more pyroclastic material)

Tephra

Fragmental material that pyroclastic material is deposited as

Mafic composition: explosive tendency

• Least explosive 

• Low silica, higher temperature (lower viscosity) '

• Low gas content 

• Least likely to produce pyroclastics 

Felsic and Intermediate composition: explosive tendency

• Most explosive 

• High silica, low temperature (higher viscosity)

• High gas content

• More likely to produce pyroclastics

Mafic (Basaltic) Composition: Volcanic Landforms

• Shield volcanoes

• Basalt plateaus

• Cinder cones

Intermediate (Andesitic) Composition: Volcanic Landforms

• Composite Cones

Felsic (Rhyolitic) Composition: Volcanic Landforms

• Volcanic Domes

• Pyroclastic Flows 

Types of volcanic landforms

• Shield

• Composite

• Cinder cone

Factors controlling volcano profiles

• Viscosity of lava 

• Angle of repose (steepest angle of stability) of pyroclastic material 

  • Tephra form steeper slopes than fine-grained ash

Shield volcano 

• Enormous 

• Gentle slope

• Lots of lava 

• Broad, low profile shape

  • reflects gentle and relatively quiet of low viscosity, basaltic lava

• Effusive (quiet) erruptions 

Composite Cone (stratovolcano) 

• Forms at convergent plate boundaries

• Fairly large, classic cone shape 

• Composed of inter-bedded lava flows and pyroclastic debris 

• Intermediate to felsic composition lava (explosive)

• More felsic - potential for volcanic dome 

Cinder cones (parasitic cones)

• Small with steep slopes

• Mostly fragments of basaltic cinders and ash (ejected due to high volatile content)

• Made of tephra

• Often on flanks of larger volcanoes

Calderas

• The top of a volcano caves in during violent eruptions 

  • Loss of magma causes the structure to cave in

• Huge amounts of pyroclastic debris and gases are released through ring fractures surrounding the summit

Toba 

Largest volcano to have affected humans, only remaining evidence is a 100km diameter caldera